In the related art, in a displacement detector which is used for roundness measurement, surface roughness measurement, shape measurement, or the like, a direction in which detection is possible is a single direction. For this reason, for example, in the case of performing bidirectional measurement of facing surfaces or opposite surfaces (back-to-back surfaces) such as a front surface and a rear surface, an upper surface and a lower surface, or an outer diameter (an outer peripheral surface) and an inner diameter (an inner circumferential surface) of a workpiece (an object to be measured), it is necessary to invert a measurement direction by rotating a displacement detector by 180, or mechanically or electrically change a direction of a measuring force (a force pressing a probe of the displacement detector against a measured surface of the workpiece by biasing of a spring or the like, that is, contact pressure).
For example, in a displacement detector disclosed in PTL 1, in order to perform bidirectional displacement detection without reversely moving the displacement detector, two sets of displacement detector composed of a bobbin (a fixed detection element) and a core (a movable detection element) in a pair are used (refer to FIGS. 3A to 3C of PTL 1).
Then, a contact part 22A or 22B of the tip of an arm 21 is pressed against a surface (a measured surface) A or B of a workpiece W, whereby the measuring force is generated by the biasing of a spring 24, and when one set of the two sets of displacement detector is in a detectable range, displacement is detected.
Further, in FIGS. 10, 11A, and 11B of PTL 1, in order to perform bidirectional displacement detection by a set of displacement detector (a bobbin 26 and a core 25), a plurality of arms 41, 21A, and 21B, a plurality of springs 24A and 24B, and a plurality of stoppers 30A and 30B are used.
Then, a contact part 42A or 42B of the tip of an arm 41 is pressed against the surface A or B of the workpiece W, whereby the measuring force is generated in the arm 21A or 21B away from the stopper 30A or 30B by the biasing of the spring 24A or 24B, and when the displacement detector is in a detectable range, displacement is detected.